Underpinning a foundation

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Underpinning a foundation

Underpinning a foundation

I am in the process of adding an addition along side of a 1950 vintage 2 story brick house. Upon excavating down to the footing, it was discovered that THERE WAS NO FOOTING! The concrete blocks were set upon a bed of mortar sitting directly upon a clay subsoil base. (Western PA area) My first instinct was to dig out and underpin the exposed area of block with concrete. Now that I think about it further, will I cause more problems? I should point out that the wall has no settlement cracks and that the new footing elevation is even with the bottom of the old block.

RE: Underpinning a foundation

You seem to have a couple of options here. Yes, you can underpin the existing wall with concrete "pillars", but very carefully. Since the existing builkding has no footing you will essentially need to provide one by eventually forming a continuous reinfofced concrete wall from the pillars. Based on my underpinning experience you should excavated a cut about eighteen (18) inches wide that extends beneath the full width of the existing wall plus about six inches. This cut should be tasken down to firm and competent bearing soils - you'll need local geotechnical adive for this determination. You may excavate these cuts at about eight foot centers along the wall. Once the cut is made the excavation should be backfilled with a poured-in-place concrete column [pillar], preferably containing reinforcing steel. When the first series of pillars have set up, repeat the process until the underpinning pillars form a continuous wall. You may still need to pour an additional reinforced concrete wall in front of the underpinning.

Alternatively, you might consider driving a series of small diameter [2, 3 or 4 inch] steel pipe piles along either the interior or exterior of the exisitng wall until they reach driving refusal. Refusal is defined as "less than 1" of pile penetration after one full minute of continuous driving with a 90 poound jack hammer." This will develop a 4,000 pound axial capacity for each pile. These piles should probably be spaced at about 3' - 0" centers because of the structural integrity of the brick wall. After the piles are driven and have met the refusal criterion they may be structurally connected to a new poured-in-plkace reinfoced concrete pile cap that is doweled into the existing wall and/or extenbded beneath the base of the wall, so that the loads are transferred into the pin-piles. This is quicker that the previous pillar underpinning and is a well proven means of providing foundation support.

RE: Underpinning a foundation

Wow,This looks like a lot more than I planned for on this project... To be sure, I must further explain the conditions before proceeding with your recommendations.

The existing house wall (without any footing) will in effect become the interior wall of a new garage---(the bottom block of this wall is below the new floorline.) For additional clarification, the entire addition will be constructed so that no additional bearing weight is added to this wall. My thoughts are to continue the new reinforced concrete footing against the outer face of block exposed at the bottom of the dig. My assumption in doing this is that it would aide in containing any further compression of the soil. I should also point out that only about a six feet area of block (that was stepped up) is now underdug(alongside of the block) for the new foundation--- the rest of the foundation block is at or below our excavation line.

As an alternate idea--- As almost all of the side wall of the existing house will be covered with the addition, would removing the brick from the sidewall have any great advantage on the weight bearing on the "footerless" foundation over leaving the brick in place?

RE: Underpinning a foundation

If you put a strip footing against the existing unsupported wall or remove the soil from one or both sides, you could induce damaging settlement in the unsupported member. That's why the other posters have correctly told you to underpin.

g7mann's "PermaJack" and pile cap system is a neat idea; I'll have to keep that one in mind for future problems -

Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

RE: Underpinning a foundation

I was always under the assumtion "IF IT AIN'T BROKE DON'T FIX IT". I again want to clarify that the subsurface clay beneath the "footerless" foundation has not allowed a single crack on the brickwork or foundation walls or of the plaster interior walls in over fifty years that the house has stood. There is no visible evidence of any subsidence ever occurring. (The current owners have lived there for over twenty years.) Be advised that the integrity of the foundation wall has not been compromised in the process of excavation--- only about 5 feet of a stepped up area at the rear corner (on one side) exposing about 6" of clay underneath. The reamaining existing wall areas are encased in the original virgin soil below our dig. We do not intend to remove any additional soils from either the inside or outside areas adjacent to this wall. Please advise.

RE: Underpinning a foundation

What is the spacing/layout of the new foundation with respect to the existing unsupported wall?

Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

RE: Underpinning a foundation

You said that, for the most part, you are not undermining the existing wall, that the wall has not settled, that there are no cracks, and that you are not adding any new load to the wall. I'm assuming that the clay is relatively stiff or hard.

What is the condition of the existing basement room on the opposite side of the wall? Could you underpin this wall from the existing basement side if you had to?

I know this is not what others here would say but, if this is your house, if you can work from the opposite side of the wall, you may want to not underpin the wall at this time. Consider building the addition and then monitoring the wall for settlement for a while (maybe a year or until you get settlement and cracks). Then, if you do have settlements, you could then underpin the wall from inside the old basement. This may be a bit more expensive and disruptive than underpinning now from outside the existing basement but it can be done.

Remember, you have had no settlements to date and you are not undermining the existing wall with your addition. You can spend some money underpinning now or you can spend a little more money underpinning and patching possible cracks later IF YOU NEED TO, and maybe you won't need to. It's your house and your gamble.

Remember though that whether or not you underpin now, when you work along side of the existing wall, do not let the ground along the wall get wet, muddy, and soft or you will settle the wall. Don't let water build up along the wall. Make sure that roof drains and downspouts are directed away from the work area. And, get the new, adjacent, wall footing built and backfilled ASAP. If you want to be a bit more conservative, build the new wall footing incrementally in several short sections so that you haven't excavated along the entire wall at one time.

If this isn't you own house, forget what I just said and get ready to underpin. If I had to underpin the wall, I would use unreinforced concrete underpinning piers. I would install the underpinning in sections, each pier about 3 to 4 feet wide. I would make the bottom of the underpinning about 6 inches to a foot deeper than the bottom of the proposed footing. I would pour the underpinning concrete to within 2.5 or 3 inches of the bottom of the existing wall and then drypack the remaining space the next morning after a pier is poured. No two underpinning piers within about 8 feet edge to edge of each other should be excavated or unpoured concurrently.

With any underpinning operation, you can expect to see some settlement. You may not get any but you should expect to see about 1/4 inch. It is not unusual.

The front wall of the addition extends 4 feet in front of (closer to the street) than the "footerless" wall. It will tie into a footer that was added 20 years ago as an extension on the front of the garage---(This footing is at the required depth, and of the proper size). The rear addition wall extends 13' beyond the existing back wall of the house.

The side portion of the addition (abutting the house) will be constructed so the new concrete block walls skirts alongside of the existing wall for 16" then turns 90 degrees inward to buttress the new foundation and to act as a separation wall to provide for a workshop at the rear of the garage. There is to be no toothing, pinning, wall-tying or attachment of the old/new walls. I intend to thoughouly clean the old wall areas that abutt the new wall and use a flexible sealer Tremco "Mono" or equal along with backer rod as a joint reinforcement/control joint.

Floor joists are to extend front to back resting upon front/rear walls, workshop wall and a steel beam W/pilaster at one side and 4"lally column placed against existing house wall set upon 18"x18" pad footing . (This footing will NOT undermine the "footerless" wall as the block of that wall extends below the dig.

Of significant importance is the fact that (as previously noted) the rear portion of the "footerless" wall (about 4 concrete blocks) steps up. The elevation at the bottom of these block is close to the top elevation of the new footer and needs to be under-pinned. I expect that the two footings that abutt one another would have to remain separated as the new foundation will settle.

Thank you in advance for your assistance.

RE: Underpinning a foundation

Thank you for sharing your alternative method of "waiting and watching". I see no reason why this method couldn't be used. As well, the new concrete floor could be poured with an expansion joint along the wall in question. If need be, "down the road" the concrete could be easily broken up and underpinning done at that time.

That said, this is not my home but my brother's residence. I will let him make the decision as to how he wants to proceed. At the very least(per g7mann's suggestion) I intend to recommend that he consult a Geotechnical PE in the area.

It is also worth mentioning and adding a note of thanks to all who responded and especially to PEinc for "red-flagging" that the area remain dry and that a decision should be made and followed up expeditiously.

RE: Underpinning a foundation

Partial underpinning is a very risky business. When underpinning a foundation is involved, most of the litigation assignments I receive are partial piering jobs. Put another way, far fewer of the fully underpinned foundations fail in comparison to those that are only partially piered.

And, yes, your brother can 'watch and wait' - but a better alternative would be to redesign the addition to avoid the problems altogether. It sounds like space is tight, and this won't be a popular suggestion. But it's something that should be discussed, anyway -

Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

RE: Underpinning a foundation

Focht3 is correct in that partial underpinning can, and usually is, much more risky than underpinning an entire structure. However, I also like the idea of waiting and watching to see what, if any, movement occurs. From your description it does not appear that the planned addition is likely to impose any SIGNIFICANT new loads so any settlement is likely to be localized and, at least initially, of low magnitude. If movement does begin to occur, then you can underpin.

Also, underpinning can, and often is, perforemd from the interior of the basement area. In many cases this is far less expensive than overexcavating down the exterior of a wall to reach the footing. Usually, the interior floor slab is only just above the footing [of bottom of the unfooted wall in this case]. Keep in mind that any "hand" work [labor] is expensive. The more you can do to minimize it the better.